Since aluminum alloys are light, have high thermal conductivity and can exhibit high corrosion resistance by appropriate treatment, aluminum alloys are used for automobile heat exchangers such as radiators, capacitors, evaporators, heaters or intercoolers. As a tube material of an automobile heat exchanger, a two-layer clad material having an Al—Mn-based aluminum alloy such as 3003 alloy as the core and a brazing filler metal of an Al—Si-based aluminum alloy or a sacrificial anode material of an Al—Zn-based aluminum alloy clad on a surface of the core or a three-layer clad material in which a brazing filler metal of an Al—Si-based aluminum alloy is further clad on the other surface of the core of such a two-layer clad material is used. A heat exchanger is generally produced by combining a tube material of such a clad material with a corrugated fin material and brazing the materials at a high temperature around 600° C.
In general, an aluminum alloy having a melting point of 600° C. or higher is used for a core material alloy of a brazing sheet, and an Al—Si-based alloy having a melting point of 600° C. or lower is used for a brazing filler metal alloy which is used for cladding. By producing a part of a heat exchanger using the brazing sheet, combining the part with other parts and heating the parts to a temperature around 600° C., only the brazing filler metal part of the brazing sheet is melted and brazed with the other parts, and a heat exchanger can be thus produced. When such a brazing sheet is used, many parts which constitute a heat exchanger can be brazed at the same time. Thus, a brazing sheet is widely used as a material of a heat exchanger.
Brazing methods which are actually used are mainly vacuum brazing method and Nocolok brazing method. For vacuum brazing method, a brazing filler metal containing an Al—Si—Mg-based alloy is used. When heated in a vacuum, Mg in the brazing filler metal is evaporated from the material and breaks the oxide layer on the surface of the material upon evaporation, and brazing becomes thus possible. However, a drawback of vacuum brazing method is that an expensive vacuum heating apparatus is required. For Nocolok brazing method, a brazing filler metal containing an Al—Si-based alloy is used. The material is coated with flux and then heated in an inert gas, and the oxide layer on the surface of the material is broken by the flux to enable brazing. However, because unevenness of coating of the flux causes a brazing failure, it is necessary to coat the necessary part with the flux evenly.
On the other hand, brazing methods which enable brazing by heating in an inert gas without using any expensive vacuum heating apparatus or flux have been proposed. PTL 1 describes a fluxless brazing method in which a product to be brazed containing Mg is coated with a carbonaceous cover and heated. In this method, Mg lowers the oxygen concentration in the carbonaceous cover and prevents the oxidation, and brazing becomes thus possible. Also, PTL 2 describes a fluxless brazing method in which a heat exchanger is composed using a clad material which contains a brazing filler metal and Mg. In this method, Mg in the brazing filler metal removes the oxide layer on the surface, and brazing becomes thus possible.
In addition, the tube shape is more complex in new heat exchangers used for recent automobiles in order to further improve the performance. Accordingly, it is now required that the materials have higher formability. The formability of a tube material has been adjusted by H14 refining type achieved by process annealing during cold rolling or by H24 refining type achieved by finish annealing after cold rolling. However, it has become difficult to satisfy the recent demand for high formability by refining alone.
In addition, when a corrosive liquid exists on the inner or outer surface of the tube of a heat exchanger, a hole may be made in the tube by pitting corrosion or the strength of pressure resistance may deteriorate because uniform corrosion reduces the tube thickness, resulting in the bursting of the tube. As a result, there is risk of the leakage of the air, coolant or refrigerant circulating inside. Moreover, when it is necessary to join a tube and a fin or to join a tube with itself for example, it is required that a brazing filler metal is provided on the surfaces. When the material is exposed to a severe corrosive environment due to snow melting salt or the like and a tube and a fin should be joined, for example in the case of the outer surface of the tube of a condenser, it has been attempted to balance the corrosion resistance and the brazing property by cladding the outer surface of the tube material with a layer having sacrificial anode effect as an intermediate layer and further cladding the outer surface with a brazing filler metal. However, as the tube shape has become complex as already described above, the corrosive liquid sometimes concentrates at a particular part, and simple cladding of an intermediate layer as in the conventional techniques is sometimes insufficient for preventing the leakage.
Techniques for improving the formability and the corrosion resistance separately have been proposed. For example, techniques for improving the formability or the electric resistance welding property of a clad material are shown in PTLs 3 and 4. However, the PTLs do not describe any means for improving the corrosion resistance of the sacrificial anode material. On the other hand, a technique for improving the corrosion resistance of a clad material is shown in PTL 5. However, the PTL does not describe any means for improving the formability of the clad material.
Specifically, regarding the clad material described in PTL 3, the electric resistance welding property of the material is improved by adjusting the mean grain size of the core material in a cross section at right angles to the longitudinal direction to 30 μm or less. With respect to the sacrificial anode material, it is defined that the area percentage of Mg2Si with a grain size of 0.2 μm or more is 0.5% or less, however, this is also means for improving the electric resistance welding property. Only the amounts of Zn and Mg are defined regarding the corrosion resistance of the sacrificial anode material, and a technique which would improve the corrosion resistance more than the conventional techniques is not described or suggested at all.
With respect to the clad material described in PTL 4, the electric resistance welding property of the material is improved by using a core material with a fibrous structure. Regarding the sacrificial anode material, it is defined that the hardness of the core material and the hardness of the sacrificial anode material are 50 Hv or more and that the ratio of hardness (sacrificial anode material/core material) is less than 1.0, however, this is means for securing the fatigue strength after braze heating. Only the amounts of Zn and Mn are defined regarding the corrosion resistance of the sacrificial anode material also in this document, and a technique which would improve the corrosion resistance more than the conventional techniques is not described or suggested at all.
On the other hand, regarding the clad material described in PTL 5, the corrosion resistance in an alkaline environment is improved by adjusting the grain size of the sacrificial anode material to 100 to 700 μm. However, only the components are defined regarding the core material, and the structure, the mechanical properties and the like thereof are not described. Also, PTL 3 does not describe or suggest the improvement of the formability at all.
Furthermore, the sacrificial anode materials described in the PTLs are all clad on the surface of a material, and it is not disclosed that the sacrificial anode materials are provided as intermediate layers between a core material and a brazing filler metal. In addition, all the brazing methods described in the PTLs are methods using flux, and a fluxless brazing method is not disclosed.